It has been a while since medical imaging modalities such as an X-ray CT (Computed Tomography), MRI (Magnetic Resonance Imaging), or ultrasound diagnostic apparatus became essential tools in a medical practice. The systems described above visualize a difference in CT value, a difference in a proton relaxation time, or a difference in an acoustic impedance, in a living body, in a form of an image, and they are called “structural imaging”, since the difference in physical properties exclusively reflects a structure (form) of a subject such as a living body.
On the other hand, the system that forms an image of portions, which have structurally the same tissue, but are functionally different from each other, is called “functional imaging”. In the functional imaging, the system that visualizes molecular biological information, i.e., the presence of biocomponent molecules, such as protein, amino acid, or nucleic acid, is often called “molecular imaging”. In the molecular imaging, a “molecular probe” that is a substance having a structure with selectivity for biocomponent molecules is often used, and in this case, a structure that can be detected with any physical means is applied to the molecular probe, in order to visualize the distribution of the molecular probe in a living body. For example, Non-Patent Literature 1 describes an example of a molecular probe when a target is a tumor. Peptide or antibody is a main molecular probe. A PET (Positron Emission Topography) device and optical imaging device can be provided as the imaging device that is specific to the molecular imaging described above.
In addition to the device specific to the molecular imaging, a system that detects and diagnoses disease, in their earlier stages than ever before, based upon a modality used in the existing structural imaging, such as MRI or ultrasound, has been developed. Among them, the system using ultrasound has characteristics not shared by the other modalities, which characteristics include that 1) the system is excellent in real-time nature, 2) the system is compact, so that it has less restriction for the use in an operation room, and 3) it can be used for not only diagnosis but also as a therapeutic tool, whereby it has been expected as an integrated diagnostic and therapeutic tool that can be used in even a place other than a large hospital.
The ultrasound used as the therapeutic tool enables a low-invasive therapy, in principle, because of spatial selectivity due to the exposure of the focused ultrasound from a site apart from a patient. A thermal coagulation therapy that increases the temperature of the target region to a protein-denaturation temperature (about 65 degrees Celsius) or more in a short period such as several seconds or several tens of seconds has received a lot of attention in recent years. It is often called HIFU therapy, since it is the therapy using high intensity focused ultrasound (HIFU) of 1 kW/cm2 or more. In the HIFU therapy, spatial selectivity of therapy is achieved by the convergence of ultrasound alone, and therefore, the HIFU therapy has a possibility that, if the region is misaligned due to a body motion, high intensity ultrasound of 1 kW/cm2 or more is exposed to a region other than the region to be treated, which causes serious adverse effect.
Therefore, a therapeutic method has been demanded that also achieves the spatial selectivity by a factor other than the convergence of the ultrasound, in order to realize safe and secure therapy. In order to attain the selectivity by a factor other than the ultrasound, a use of medical agent has been studied, and in particular, a therapeutic method using bubbles such as microbubbles that are frequently employed as an ultrasound contrast agent has been highly expected. For example, it has been found that, as described in Non-Patent Literature 2, an apparent absorption coefficient at tissues exposed to ultrasound increases due to the presence of the microbubbles. As the citation involved with the HIFU therapy and microbubbles, there are Non-Patent Literatures 3 and 4, and Patent Literatures 1 and 2, in addition to the above-mentioned Literature.